Method and apparatus for controlling the load-speed characteristics of moving systems



June 21, 1932. DQYLE 1,864,189

METHOD AND APPARATUS FOR CONTROLLING THE LOAD SPEED CHARACTERISTICS OF MOVING SYSTEMS Filed June 11, 1928 2 Sheets-Sheet l [NVE N TOR ATTORNEY June 21, 1932. DOYLE 1,864,189

METHOD AND APPARATUS FOR CONTROLLING THE LOAD SPEED CHARACTERISTICS OF MOVING SYSTEMS Filed June 11, 1928 2 Sheets-Sheet 2 r viii? 5 Z 4) l1lllllll+llnlli7llll '"5 g. g u g 2 n E z E o a o a, J

INVENTOR BY I Mu -6&1

2&1 A TTORNE Y Patented June 21, 1932 UNITED STATES PATEN'I'. OFFICE EDGAR D. DOYLE, F PHILADELPHIA, PENNSYLVANIA, ASSIGNOR. TO LEEDS &

NORTHRUP COMPANY, OF PHILADELPHIA, PENNSYLVANIA, A CORPORATION 01" PENNSYLVANIA METHOD AND APPARATUS FOR CONTROLLING THE LOAD-SPEED CHARACTERISTICS OF MOVING SYSTEMS Application filed June 11,

. My invention relates to a method and apparatus for controlling the load-speed characteristics of one or more moving systems.

In accordance with my invention there is utilized a speed-responsive device and a loadresponsive device which jointly elfect a control to modify the normal load-speed characteristic of a moving, work-producing unlt; more specifically, a frequency-responsive device and a load-responsive device, as a watt meter, jointly effect control of the prime mover of an alternator,v as a turbine, by controlling the fiow of steam to the blades of the turbine rotor.

Further in accordance with my invention, in a group of moving systems, as for enample, of alternators operating in parallel, having a common load, there are utilized control mechanisms to effect a predetermined distribution of load between the several units, more specifically and preferably, the. control. mechanisms are jointly responsive to change of load and frequency to effect distribution of the total load between them in a predetermined manner, the load on the individual units being a function of the total load;

My invention resides in a method andapparatus of the character hereinafter described and claimed. Y

For an understanding of my method, and for an illustration of someof the various forms my invention may take, reference is to be had to the accompanying drawings in whlchz Fig. 1 is a diagrammatic view of apparatus suitable for practicing my method and embodying my invention in one of its forms.

Fig. 2 is a fragmentary perspective view of 1928. Serial a... 284,369.

whose load-frequency characteristic is modified in any desired manner.

Its terminals are connected respectively to the variable contacts 1 and 2 constituting conjugate points of aWheatstone bridge, whose other conjugate points are the points of engagement of contacts 3 and 4 movable along the slide wires or adjustable resistances S and S1, respectively. In the first bridge arm is included a condenser C shunted by the noninductive resistance r; in series with that portion 74 of the non-inductive resistance S lyin to the left of contact 3 and that portion 0 non-inductive resistance r12 above contact 1. In the second bridge arm are included the condenser C2, non-inductive resistance 1'5 and that portion of the non-inductive resistance r13 above .contact 2 and thenon-inductive resistance S to the right of contact 3. In the third bridge arm are incduded the non-inductive resistance 16, that portion of 1'12 below contact 1 and the portion a? of the slide-wire resistance S1 to the right of contact 4. In the fourth bridge arm are the resistances 1 8 constituting the remainder of the resistance S1 to the left of contact4, that portion of 113 below contact 2 and the non-inductive resistance r9. 7

In case the resistances S and S1, which may be in the form of simple wires or in the-form of helices along which contacts 3 and 4.- may move, are not of themselves exactly correct in value, they may, as well understood in the art, be shunted by resistances such as 110 and 1'11, respectively, to insure between their terminals exactly the correct or desired amount of resistance.

Resistance units 7'12 and 113 may be so constructed that movement of contacts 1 and 2 effects a uniform increase or decrease of resistance. However, either or both, may be so constructed that equal incrementsof contact movement effect dissimilar or varying I increments of resistance for a portion, or portions of, or the whole of, the range of contact movement, for a purpose hereinafter described.

It is desirable that all of the resistances aforementioned shall have small or substantially zero temperature co-eflicient, to avoid influence of temperature upon the measurements.

While the source A is in one conjugate conductor of the bridge, in the other, between the contacts 3 and 4, is connected the movable coil 20 of a galvanometer having the defiect ing pointer or needle 19. The coil winding may, as well understood in the art, be shunted by a resistance T20, of suitable magnitude. The alternating magnetic field of the galvanometer is produced by current from source A traversing the galvanometer field winding or coil 7, in series with which may be connected the resistance r14.

The phase angle of the current exciting the galvanometer magnet winding f with respect to the impressed electromotive force may change or vary from various causes, such as, for example, temperature changes of winding f, etc. Such a change of phase of the field exciting current will introduce error in themeasurements unless means are provided to make possible a true exciting current" of the galvanometer. To this end it is essential that three ratios shall be equal for all frequencies within the frequencies to be employed. This equality of ratios is expressed by the following equation:

111.42 1 r2 d2 b v in which T1 and (ii are, respectively, the re-, sistance and reactance components of one arm of the bridge, as, for example, the aforesaid first arm including C1, 1-, 14:, etc. T2 and (12 are, respectively, the resistance and reactance components of another arm of the bridge, for example, the aforesaid second arm, including r3, 1 6, G2 etc.; are, respectively, the resistances or impedances of the other arms of the bridge.

Inthe illustrative example, where capaci tative reactances or condensers are employed, it is desirable that they be low loss condensers, to avoid phase angle or capacity changes with temperature changes, and are preferably mica condensers of the best type.

The combination of a condenser, as G1, with a shunting resistance, as 1", is the equivalent and has the characteristics of a condenser having considerable losses, which may be expressed in terms of an equivalent resistance which changes in magnitude with changes in frequency. Accordingly, the

aforesaid resistance T1 is an apparent, fictitious or equivalent alternating current resistance, which changes in magnitude with changes in frequency, and comprises not only the resistances 7'. 4'4. and a variable portion of r12, but, in addition, the apparent resistance corresponding with the losses, if any, of the condenser 01, per se, and of the combination of the condenser G1 with its shunting resistance. The aforesaid resistance r2 correbalance of the bridge regardless of the shifting of phase of the fieldand a and 6.

sponds with the sum of the resistances 1'3 and T6 and the apparent resistance, variable with frequency, due to losses of condenser C2, if' any.

From equations 'wcr2 'mwa r f d2=$ w=omega well known to those skilled in the alternating current art, and in which w (omega) is 2n frequency, w and y .are, respectively, the capacities of condensers Cl and C2, and 1', 1'4 and 9 12 are the magnitudes of 1', 74: and that portion of 712 included in the first arm of the bridge, are determinable the proper values 0 the several resistancesa-nd capacities to be utilized in an arrangement such as indicated in Fig. 1, for use withina desired range of frequencies, it being understood that contacts 3 and 4 are moved in unison with each other because attached to the 'same movable member "C.

For a given position of contacts 1 and Q, the contacts 3 and 4 take a different position for each different frequency, in order to-balance the bridge for that frequency, in which event the galvanometer coil 20 is in its zero or nondeflecting position.

From the foregoing it will be understood that the function of the slide-wire S1 is to change the ratio of the resistances in the third and fourth arms of the bridge in proportion to the change in the ratio of effective resistance and reactance in the first and second arms of the bridge in order that the equality of ratios expressed" in equation (1) shall obtain, whereby the bridge may be truly balanced regardless of changes gle in the galvanometer field.

Referring to Fig. 2, M is an electric motor or other suitable source of power rotating the shaft 5 at-xsubstantially,constant speed. A lever 6 is pivoted at its upper end on a horizontal axis and has pivoted thereto on a horizontal axis at its lower end the arm 7, on each end of which is a shoe 7a, of cork or the like, frictionally engaging the rim 8 of the clutch disk or wheel 9, secured upon the shaft 10 of'the movable structure or system. Secured upon the shaft 5 is a cam 11, which periodically engages the lever 6 and moves it outwardly, in opposition to a spring, not

shown, thereby lifting the shoes 7a from the rim 8, the spring returning the shoes into engagement with the rim after predetermined rotation of the cam 11. Upon the shaft 5 is of the phase ani the member 15 pivoted on a horizontal axis at 16. Upon the member 15 is secured the member 17, whose upper edge 18 is inclined outwardly from the center. Disposed immediately above the edge 18 is the aforesaid needle or" pointer 19 of the alternating current galvanometer whose-coil is 20. At the opposite ends of the member 17 are the abutments 21 for limiting the deflection ofthe needle 19. Directly above the needle 19 and beneath which it normally freely swings are the edges 22, preferably straight and horizontal, upon the members 23, 23, pivoted at 24, 24, and extending toward each other, leaving a gap of sufficient width between their inner ends for free entry of the needle 19 when in balanced or zero position, corresponding with a balance of the WVheatstone bridge of the character shown in Figs. 1, 4 and 6. The needle 19 normally swings free- 1y between the edge 18 and the members23, 23, which latter have downwardly extending arms biased toward each other by the spring 26. Attached to the lower end of the arm or lever 6 is a triangular plate 27 carrying the pins 28, 28, co-operating with the lower ends of the members 23, 23. At opposite ends of the arm 7 are the lugs 29, 29, adapted to be engaged by the cams 30, 30, similar in shape and similarly positioned and secured upon the shaft 5. 1

As the galvanom'eter needle 19 deflects in the one direction or the other, the shaft 10 will be rotated in the one direction or the other, and to an extent corresponding to the extent of the galvanometer deflection. For example, when the needle 19 deflects to the right, Fig. 2, it is clamped, due to periodic vertical movement of member 15 by cam 12,

between the inclined edge 18 and the lower edge 22' of theright-hand member 23. causing the member 23"to be tilted in a clockwise direction about its pivot 24, thereby pushing the right-hand pin 28 on plate 27 and so tilting themovable driving clutch arm 7 in a clockwise direction whilecam 11 is holding shoes 7a, 7 a from the rim 8 of a clutch wheel 9, the angular movement of the member 7 being dependent upon the degree of deflection of needle 19. The cam 11 soon thereafter allows the shoes to grip again the rim 8 of wheel 9 and soon thereafter the left hand cam 30 engages the ear .29 of arm '7, which has been elevated, pushing it downwardly-due to rotation of shaft 5, to restore it to the horizontal position indicated, but in so moving back to normal position, the clutch member rotates the disk 9 and shaft lOafiixed thereto in a counter-clockwise direction.

Secured upon the shaft 10 is a disc 31 of insulating material, carrying upon its periphery the aforesaid resistances or impedanees S, S1, which in this example, as indicated in Fig. 3, are rotated, while their co-acting contacts 3 and 4 are stationary. Secured upon the shaft 10 is a second disc 32 carrying the arcuate contacts 33 and 34, with which co-a'cts.

the stationary contact or brush 35. The discs 31 and 32 are adjustable to any suitable angles witlrr'espect to each other and with respect to the shaft 10 by suitable set screws 36 and 37.

Secured upon the shaft 10 is the grooved pulley wheel 38, around which is wrapped the cord 39, which passes over the idler rollers 40 and has attached thereto the pen or marker 41 for drawing upon the record sheet termined by the impedanoes in the arms of the bridge. For a definite frequency, the bridge is balanced and no movement of the control shaft 10 is effected, Whereas for other frequencies the bridge is unbalanced and the shaft 10 is rotated in a sense depending upon the sense of unbalance.

A turbine or other source of power, or generically a rotating system, T, drives the alternating current generator or source A related to the bridges of Figs. 1, 3 and 4. The source A in this instance may be a small alternating current generator driven by the rotary structure Tjthrough the shaft 44, or it may be .an alternating current generator of large capacity for supplying current to any suitable load connected to the supply circuit conductors 45 and 46, in which case'the frequency or speedof T and A may be controlled by controlling the opening of the valve V controlling the rate of supply of motive fluid or steam through the pipe 47 to the turbine T. In this case the brush 35, with contacts 33 and 34, controls the reversible electric motor M1 receiving current from any suitable source, as 48. When the disc 32 is in the position indicated in Fig. 1, the mot-or M1 is deenergized, and this condition corresponds with the position of balance of the VVheatstone bridge. Should the bridge be unbalanced, the galvanometer needle 19 is deflected, causing rotation of shaft 10 to an extent corresponding with the extent of unbalance, the contact 33, for example, will be brought into engagement with the eontact 36, energizing the motor M which runs in a certain direction, rotating the shaft 49, through any suitable great gear reduction, generally represented by the pulleys 50, 51 and belt 52, thereby causing the member 53 to advance upwardly along the threaded shaft 49, carrying the abutment 54 upward- 1y, with resultant tendency to compress the spring 55, which opposes'the fly ball governor 56, driven by the turbine or device T, to partially actuate the valve V, changing the speed or frequency. Y

The controller mechanism is similar to that described in U. S. patent to Leeds No. 1,125,699.

Thus far described, the position of member C depends solely upon the frequency of current generated by the alternator A. That feature is claimed in copending application Serial No. 18,79 i filed March 27, 1925, and is not, per se, the invention herein claimed. Referring to Fig. 3, unit No. lis an apparatus comprising a self-balancing bridge of the type described except that resistances T12 and 713 have been omitted. This control unit serves to maintain the frequency of the alternator associated therewith at a desired value in any desired manner, for example, by controlling the flow of steam to a turbine coupled thereto. The specific circuit and control arrangement is more fully described in the aforesaid copending application. It is to be understood that the speed of this unit may be controlled in any other suitable or satisfactory manner or with other apparatus, as in the features of my invention involving this unit, the specific manner in which speed control is effected is immateriai and the foregoing is for the purpose of explanation only.

Reverting to Figs. 1, a and unit No. 2 of Fig. 3, when the contact 2 is moved manually along resistance 713, the constants of the second and fourth bridge arms above identified are altered and the contacts 3 and 4, mounted on member C, assume a new position of balance for any given frequency; in efiect, the calibration of. the apparatus has been changed. v v

By observing the indication of 21 watt meter and shafting the position of contact 2 along resistance 113, an operator may alter the characteristic load-frequency curve of the unit comprising alternator A and turbine T, or their equivalents, so that instead of being of the steadily drooping nature as hereinafter explained, it may be of any desired shape. It is more desirable, however, only to efiect a rough control in such manner and to change the calibration in a manner now described.

The current and voltage coils of a watt meter W are connected, respectively, in the conventional manner, in series with, and in shunt to, the output circuit of the alternator A. A shaft 57, driven by the movable element of the watt meter through suitable mechanism, as, for example, gear 58, gear sector 59 and shaft 60, moves the contact 1 along resistance. 912 so that the amount of power supplied by alternator A uniquely determines the position of contact 1 and therefore to a predetermined extent affects the calibration of the apparatus. Consequently, for any given'frequency, the point of the balance of the bridge or the position of contacts 3 and 4, depends upon the load carried by the alternator.

It is to be understood that the extent of movement of contact 1 may bear any desired relation to the extent of movement of the rotatable element of the watt meter and, fur-. ther, that the ratio between the increments of movement of the movable contact 1 and the rotatable or movable watt meter element may vary throughout the range of movement. As before stated, the resistance 712 may be constructed so that equal increments of movement of contact 1 result in either equal or unequal changes of resistance. By suitably designing the mechanical connections between the movable element of the watt nieter, or suitably constructing resistance 112, or both, calibration of the apparatus may be changed to any desired extent for a change from a given load to another given load and, further, the calibration may be affected to difierent extents by equal increments of load throughout the range of the apparatus. The speed-load curve of the moving system will, as explained hereinafter, be correspondingly modified.

For the purpose of explanation, it will be assumed that in Fig. 1 the frequency and load are normal. If the load is increased, there will be a fall in the frequency which unbalances the bridge, causing a current to flow through galvanometer coil 20, deflecting needle 19 and effecting a movement of control shaft 10 through the mechanism above described. The degree of unbalance is determined not only by the change in frequency, but also by the change in position of contact 1, due to the increased load, such change in position of contact 1 being proportional in amount to the increment of load increase and being in such direction that balancing of the bridge occurs at a frequency lower than normal.

Contact 33 on control 32 is brought into engagement with fixed contact 35, closing the circuit of reversible motor M1, to operate valve V, in such a sense to permit an increased flow of steam to turbineiT, tending to increase the speed of T and the frequency of the output of A.

The lower frequency attained when the bridge is balanced for the new position of contact 1 as effected by watt-meter W, is

'maintained by the'control mechanism as above described for normal load and frequency until the load again varies or the contact 2 is manually adjusted. As the controller does not tend to maintain the frequency constant irrespective of load but varies with load, it will be termed, for the purpose of convenience, a load-biased frequency controller.

Referring to Fig. 4, the bridge circuit is identical with that of Fig. 1, except that the resistance r12 and contact 1 have been omitted. In this form of my invention, the calibration of the apparatus is-not alfected by the alternator so that, if desired, a chart 42 may record directly in terms of frequency or revolutions per .unit of time, provided that the position of contact 2 remains unchanged. The contact 2 may be provided with a scale so that if moved, a definite indicated correction may be added or subtracted from recorded frequency.

To effect a modification of the load-speed curve, the movable element of the watt-meter W changes the position of contact with respect to contacts 33, 34, carried by the control disc 32 through any desired connecting mechanism. If an arrangement as shown in Fig. 1, for moving contact 1, is used, the extent of movement of contact 35 is in direct proportion to movement of the load element. By utilizing an arrangement as in Fig. 4, in which the watt-meter shaft 57 drives a cam 61, which may be replaced by acam of different contour,'or which is itself of adjustable configuration, in engagement with a cam follower 62, connected to shaft 63, on which contact 35 is fixedly attached, and by properly designing or adjusting the shape of cam '61, it is possible to secure a load-speed curve of a desired outline.

When the control disc 32 and contact 35 are in the position shown in Fig. 4, the frequency of, and the load upon, alternator A are normal. Upon increase of load, the line frequency drops unbalancing the bridge and effecting movement of control shaft 10. Simultaneously the contact 35 is rotated by the movable element of watt-meter W to an extent determined by the increase of load, the mechanical connectionsincluding cam 61 be tween the contact and the watt-meter element, etc. The circuit through motor M1 is completed and as above described, the valve V is operated to tend to increase the speed of turbine T. The lower frequency attained when the bridge is balanced for the new position of contact 35' as effected by watt-meter W, is maintained substantially constant until the load again varies or'contact 2 is manually or otherwise adjusted. This type of controller is also conveniently termed a load-biased frequency controller.' The samecontroller may be utilized {simultaneously to control a plurality of alternators by having shaft 10 rotate a corresponding number of control discs 32 each co-acting with a contact 35 whose position is determined by the load upon the alternator associated therewith.

In Fig. 3, the alternators A, A1, and A2, located at the same or isolated points are connected in parallel to circuit 45, 46, each of which is driven by a prime mover whose speed is controlled, as turbines T, not disclosed in this figure but similar to that to left of line BB in Fig. 1. Control unit N o. 1 associated with alternator A1 is a normal, or simple frequency controller, as described hereinbefore and in the aforementioned application, whereas control units 2 and 3, associated with alternators A and A2, respectively, are load-biased frequency controllers or controller-recorders.

The contact 1 of Figs. 1 and 3 or the contact 35 may, as above described, in each of the control units produce unequal increments of biasing effect upon the alternator system, for equal increments of change of load to modify the shape of the characteristic loadresponse curve in a desired manner, or the increments of biasing efi'ect maybear a linear relation to the change of load. For example, curves a and a2 of Fig. 6 are lines of different but constant slope, whereas in curve a3 regions of slight slope are connected by a short portion of great slope.

Assuming that all -alternators connected to the line are load-biased, as A and A2, the system load may be distributed between the individual machines in a desired manner by suitably modifying the load-speed curve of the units. Obviously two or more alternators operating in parallel have the same frequency and the distribution of load between the machines andthe line frequency will depend upon the characteristics of all machines as produced by the load-biased frequency controllers. For example, referring to Fig. 7, in which lines a and a2 represent, respectively, the load-frequency curves of alternators A and A2 when the system load is L1, alternator A carries the entire load, the frequency under this condition being f1; when the system load' increases, so that, for example, the frequency is f2, alternator A carries load L2 and alternator A2 carries load L3, the system or station load being, of course, the sums of. loads L2 and L3. The alternator A under the last condition carries the larger part of the system load but when the total load increases to a, value that the frequency decreases to a value f3, alternator A2 carries the large part L5 of the load and alternator A the remainder. It is a feature of the arrangement that the load on the individual alternator is a function of the total load. The load-frequency curves of systems of inherently different characteristics are substantially identical when associated with controllers of practically similar predetermined characteristics.

When an alternator is provided only with simple frequency control, as alternator A1,

Fig. 5, and another or other alternators are load-biased, as A and A2, under normal conditions alternator A1 is able to maintain the predetermined frequency and the individual loads upon the load-biased machines remain unchanged and are represented by L6 and L7. If the system load is so greatthat the additional load cannot be supplied by alternator system Al, the frequency control is ineffective and the frequency falls permitting the load-biased machines to take on additional load at the expense of system frequency. Upon restoration of normal conditions, the excess load is shifted'back to alternator A1 having simple frequency control. The load upon the load-biased machines can, of course, be changed by changing the calibration of the bridge circuit, i. e. by shifting contact 2 along resistance 713.

It is to be understood that my invention is not limited to turbo-alternator systems, or electrical power systems generally.

What I claim as my invention is:

1. A system comprising a movable member, driving means therefor, means adjustable to vary the speed of said driving means, means responsive to the speed of said member, means responsive to the load upon said member, and governor means responsive to the speed of said driving means to control said adjustable means, affected by said load responsive means and said first speed responsive means.

2. An alternating current supply system comprising a group of alternators operating in parallel, driving means for each alternator, a plurality of means each adjustable to vary the input energy to one of said driving means, means responsive to system frequency for regulating one of, said adjustable means whereby the associated alternator carries the normal increases of system load and maintains the system frequency constant despite said normal increases of load, and a plurality of means each responsive to system frequency and the individual load upon one of the-alternators to regulate the other adjustable means whereby increases in load lIlCXCQSS of maximum normal increase are shared by said other alternators and the system frequency held constant at a subnormal value determined by the system load.

3. An alternating current supply system comprising a group of alternators operating in parallel, driving means for each alternator, means adjustable to'vary the speed of each of said driving means, a plurality of means each responsive to the system fI'G'. quency, a plurality of means each responsive to'the load upon an alternator of said group, and a plurality of speed-responsive means for regulating the speed adjusting means of said alternators, and each controlled jointly by one of said frequency responsive means and one of said load responsive devices.

4. An alternating current supply system comprising a group of alternators operating in parallel, driving means for each alternator,

means adjustable to vary the input energy to said driving means, and a control system-for effecting a predetermined distribution of load between said alternators and a predetermined redistribution of load upon a change in system frequency irrespective of the load-speed characteristics of said units comprisin means responsive to the system frequency or controlling each of said adjustable means, and means responsive to the loads upon the individual alternators to modify in accordance with a predetermined load-frequency characteristic the frequency control upon each of said adjustable means to effect a predetermined relation between the individual alternator loads for each different system frequency.

5. In a system of the character described, apparatus forming part of said system, and means providing for control of said apparatus in accordance with the joint effect of speed conditions of said system and load conditions with respect to said apparatus; said control means including a galvanometer having an element movable to different positions from a given position, and means associated with said-element and operable in accordance with the degree of departure of the same from said given position.

6. In a system of the character described,

apparatus forming part of said system, and

means providing for control of said apparatus; said control means including a balanced electrical circuit, means providing for unbalancing" of said circuit in degree corresponding to changes in load conditions on said apparatus, and means providing for unbalancing of said circuit in degree corresponding to changes in speed conditions of said system.

7. In a system of the character described, apparatus forming part of said system, and means providing for control of said apparatus in accordance with speed conditions of said system and load conditions with respect to said apparatus, said means including a VVheatstone bridge.

8. In a system of the character described, apparatus forming part of said system, means operable to control said apparatus in accordance with the joint effect of speed conditions of said system and load conditions with respect to said apparatus, and cam means forming part of said control means.

9. In a system of the character described, an electrical distribution line, a dynamo electrical machine connected to said line, and means providing for control of said machine; said control means including a balanced electrical circuit, means providing for unbalancing of said circuit in degree corresponding to changes in load conditions on said machine, and means providing for unbalancing of said circuit in degree corresponding to changes in speed conditions of said system.

10. A system comrising an alternator, driving means therefor, means adjustable to vary the speed of said driving means, a governor driven by said driving means and I controlling said adjustable means, and means jointly responsive to the speed of the alternator and to the load thereon to vary the setting of said governor.

11. A system comprising an alternator,

driving means therefor, a governor driven by said means for controlling the speed thereof, an electro-mechanical system controlling the setting of said governor and responsive to changes in frequency from a predetermined frequency, means manually adjustable to determine the frequency at which said electromechanical system is balanced, and means responsive to the load of said alternator for changing the frequency at which said electromechanical system is balanced.

12. An alternating current supply system comprising a group of alternators operating in parallel, driving means for each alter-v nator, governors driven by said driving means for controlling the input energy supplied thereto, and means for changing the governor settings for different values of total load to effect for each value thereof a predetermined distribution of load between the alternators of said group comprising a plurality of electromechanical systems each Varying the setting of the associated governor when the frequency is other than a predetermined frequency at which the electro-mechanical system is balanced, and a plurality of means each responsive to the load of an alternator for varying the frequency at which the associated electromechanical system is balanced.

13. An alternating current supply system comprising an alternator, and means for controlling said alternator comprising a balanced electrical circuit, means for varying the input energy to said alternator when the system frequencyis other than the frequency at which said circuit is balanced, and means responsive to the load upon said alternator for varying the frequency at which said circuit is balanced.

EDGAR; n. DOYLE. 

